(a) Photograph of the split-and-delay unit (shown at zero delay): (1) Multilayer mirror M2 which can be moved with a (3) short-range or (4) long-range linear stage with respect to the (2) fixed multilayer mirror M1. (5) Fluorescent Ce:YAG screen, which can be introduced into the beam path for monitoring the beam(s). (b and c) Camera view of the FEL-beam(s) on the Ce:YAG screen: (b) intensity profile without SDU, (c) intensity profile at active SDU and 50%/50% split-ratio at the full delay. Note that a lateral offset of the split-beam with respect to the incoming FEL results from the z-shape of the DESC beam path (offset 1). In addition, the finite incident angle α leads to a time-delay-dependent gap in between pump- and probe-beam (offset 2). The ellipsoidal shape of the beam profiles is caused by a 45° angle of incidence on the screen. The ring structures overlaying the intensity profiles originate from beamline-apertures in the FEL tunnel. The bright oblique lines indicate the edges of the screen (edge length = 25 mm).